1. Field of the Invention
This invention relates to rudders for naval and commercial vessels and more particularly relates to a rudder and method for minimizing early cavitation and related ship vibration and for thus improving maneuverability thereof.
2. Review of Related Art
Rudders provide a vessel with directional stability, control, and maneuverability. Ship rudders are generally vertical hydrofoils with symmetric profiles, i.e., horizontal cross-sections symmetric about the profile longitudinal centerline (chord) as shown by conventional rudder 26 in FIGS. 1-3. Rudders generate hydrodynamic lift forces to produce ship turning moments for maneuvering and directional control. The lift force produced varies with the rudder's angle of attack (angle of the rudder chord relative to the onset flow angle) and the incoming flow velocity (velocity of flow into the rudder). The effectiveness of rudders in performing their hydrodynamic functions is proportional to the square of the incoming flow velocity, thus, rudders are generally placed behind propellers where the incoming flow is accelerated by the rotating propeller.
In existing rudder design practice, virtually unchanged since the 1940's, naval architects and marine engineers initially determine the overall rudder size (rudder chord, i.e., longitudinal distance from leading edge to trailing edge, and rudder span, i.e., vertical distance from root to tip) based on consideration of the ship's required turning diameter and thus required turning moment. Rudder section profiles are then selected from sections of the National Advisory Committee for Aeronautics (NACA) such as NACA 4-digit profiles (e.g., NACA 0020) or from alternative section profiles such as TMB-EPH (elliptic-parabolic-hyperbolic) sections.
These conventional rudders are located either along the ship centerline directly behind the propeller, in the case of a single shaft designs, or behind the propellers and positioned symmetrically about the ship's longitudinal centerline in the case of a multiple shaft designs. Thus, the rudders are located in the propeller slip stream or trailing wake, i.e., accelerated flow in the propeller wake. The propeller slip stream is a region of highly complex flow having axial, tangential and radial flow components. The rotating propeller accelerates the flow and sheds vortices that impinge on the rudder surface. Consequently, a rudder operating behind a rotating propeller encounters, in addition to the flow at substantially the ship's velocity (ship wake), vortices shed by and induced velocity generated by the propeller (propeller trailing wake). Depending on the propeller's size, hydrodynamic loading and position relative to the rudder, the incoming flow to the rudder exhibits induced flow angles (onset flow angles) that can vary longitudinally along the chord of the rudder (chordwise) and vertically along the span of the rudder (spanwise).
Because the propeller accelerates and rotates the flow into the rudder and because the vortices shed from the propeller impinge on the rudder surface, the flow entering the rudder plane exhibits larger onset flow angles (angles of incoming flow relative to the ship longitudinal centerline) than would result without the propeller present. Due to the complexity of flow field in the propeller slip stream, the influence of accelerated cross-flow induced by the propeller onto the rudder is not considered in existing rudder design practice. The simple conventional rudder design practice, however, results in problems in terms of rudder performance.
As a result of the propeller generating non-zero onset flow angles in the rudder plane, rudders with symmetric profile sections placed parallel to the ship centerline experience non-zero angles of attack and generate hydrodynamic lift and induced drag, even if the ship is operating in a straight ahead course. Because of non-zero onset flow angles, suction pressure peaks (highly decreased pressure) occur at or near to the leading edge of the rudder. Surface cavitation can be predicted from the pressure distribution on the rudder surface. Cavitation inception occurs when local pressure drops to or below the local vapor pressure of the flowing fluid. Therefore, in areas of suction pressure peaks early cavitation inception can occur on the rudder.
Because of the leading edge suction pressure peaks produced by the large propeller induced angles of attack experienced by conventionally designed rudders, early cavitation inception occurs at the rudder leading edge. Viewing the rudder and propeller from behind the ship, a right-hand rotating (clockwise rotating) propeller will produce flow having velocity components directed to the right of the propeller centerline while left-hand rotating (counterclockwise rotating) propellers will direct components of flow to the left of the propeller centerline. Therefore, depending on the direction of propeller rotation and, thus, the direction of induced flow angle into the rudder, cavitation may occur on either the inboard or outboard side of the rudder in twin shaft designs.
Early rudder cavitation results in an undesirable compromise in hydrodynamic and acoustic performance of the vessel. Specifically, rudder cavitation induces unsteady hydrodynamic forces, vibration, and rudder erosion. The existence of non-zero onset flow angles also reduces the available rudder angles for avoiding rudder stall at low speeds. Furthermore, the induced drag from the finite lift force and the form drag from the rudder cavity create additional ship resistance.
Ship rudders are subjected to propeller induced velocities and induced flow angles that vary along the rudder span and chord. Because of non-zero onset flow angles, a suction pressure peak is formed at or near the leading edge of the rudder where early cavitation occurs. It would, therefore, be advantageous to both hydrodynamic and acoustic performances to alleviate the occurrence of suction pressure peaks and early cavitation. Thus, there is clearly a need to improve rudder cavitation inception speed (i.e., delay cavitation inception) and to improve hydrodynamic and acoustic performances of rudders.